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# Wednesday, May 4 th : “A” Day Agenda (1:05 release) Collect Take-Home Quiz Section 15.2: “Acidity, Basicity, and pH” Self-ionization constant of H

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• Wednesday, May 4 th : A Day Agenda (1:05 release) Collect Take-Home Quiz Section 15.2: Acidity, Basicity, and pH Self-ionization constant of H 2 O, K w, neutral, pH, indicator Homework: Section 15.2 review, pg. 547: #1-3, 5, 7-12 Concept Review: Acidity, Basicity, and pH Looking Ahead: Titration Lab: Thursday, May 12 th Chapter 15 Webquest/concept review due: Wed, May 18 th
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• Self-Ionization of Water Water can act as an acid and as a base. This means that a water molecule can either give or receive a proton. Base Acid CA CB A pair of water molecules are in equilibrium with two ionsa hydronium ion and a hydroxide ionin a reaction known as the self-ionization of water.
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• Self-Ionization of Water The equilibrium expression for this reaction is simply K eq = [H 3 O + ][OH ] This equilibrium constant, called the self- ionization constant, is so important that it has a special symbol, K w. Base Acid CA CB
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• Self-Ionization of Water In pure water, the two ions must share the same concentration. Experiments show that this concentration is 1.00 10 7 M at 25C. [H 3 O + ] = [OH ] = 1.00 10 7 M K w = [H 3 O + ] [OH - ] K w = 1.00 X 10 -14 Base Acid CA CB
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• Self-Ionization of Water The product of these two ion concentrations is always a constant. Anything that increases one of the ion concentrations decreases the other. If you know one of the ion concentrations, you can calculate the other.
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• Self-Ionization of Water The concentration of hydronium ions (H 3 O + ) in a solution expresses its acidity. The concentration of hydroxide ions (OH - ) in a solution expresses its basicity.
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• Sample Problem A, pg. 541 Determining [OH - ] or [H 3 0 + ] using K w What is the [OH - ] in a 3.00 X 10 -5 M solution of HCl? HCl is a strong acid and ionizes completely: HCl (g) + H 2 O (l) H 3 O + (aq) + Cl - (aq) 3.00 X 10 -5 M 3.00 X 10 -5 M Because 1:1 ratio, [HCl] = [H 3 O + ] = 3.00 X 10 -5 M K w = [OH - ] [H 3 O + ] = 1.00 X 10 -14 1.00 X 10 -14 = [OH - ] (3.00 X 10 -5 M) [OH - ] = 3.33 X 10 -10 M
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• Additional Practice What is the [H 3 O + ] in a solution of NaOH whose concentration is 3.75 X 10 -2 M? NaOH is a strong base and ionizes completely: NaOH (s) Na + (aq) + OH - (aq) 3.75 X 10 -2 M 3.75 X 10 -2 M Because 1:1 ratio, [NaOH] = [OH - ] = 3.75 X 10 -2 M K w = [OH - ] [H 3 O + ] = 1.00 X 10 -14 1.00 X 10 -14 = 3.75 X 10 -2 M [H 3 O + ] [H 3 O + ] = 2.67 X 10 -13 M
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• pH and Acidity When acidity and basicity are exactly balanced such that the numbers of H 3 O + and OH ions are equal, we say that the solution is neutral. Pure water is neutral because it contains equal amounts of the two ions.
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• Measuring pH The letters p and H stand for power of hydrogen. pH can be calculated by the following mathematical equation: pH = log [H 3 O + ]
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• Measuring pH Because of the negative sign, as the hydronium ion concentration increases, the pH will decrease. A solution of pH 0 is very acidic. A solution of pH 14 is very alkaline. A solution of pH 7 is neutral.
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• Measuring pH The pH equation may be rearranged to calculate the hydronium ion (H 3 O + ) concentration from the pH. [H 3 O + ] = 10 pH
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• pH values of some common materials
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• Sample Problem B, pg. 544 Calculating pH for an Acidic or Basic Solution What is the pH of (a) a 0.00010 M solution of HNO 3, a strong acid, and (b) a 0.0136 M solution of KOH, a strong base? (a): HNO 3 is a strong acid and ionizes completely: HNO 3 (g) + H 2 O (l) H 3 O + ( aq ) + NO 3 - (aq) 0.00010 M Because 1:1 ratio, [HNO 3 ] = [H 3 O + ] = 0.00010 M pH = -log [H 3 O + ] pH = -log (0.00010) pH = 4.00
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• Sample Problem B continued What is the pH of (a) a 0.00010 M solution of HNO 3, a strong acid, and (b) a 0.0136 M solution of KOH, a strong base? (b): KOH is a strong base and ionizes completely: KOH (s) K + (aq) + OH - (aq) 0.0136 M 0.0136 M Because 1:1 ratio, [KOH] = [OH - ] = 0.0136 M K w = [OH - ] [H 3 0 + ] = 1.00 X 10 -14 1.00 X 10 -14 = (0.0136 M) [H 3 0 + ] [H 3 0 + ] = 7.35 X 10 -13 pH = - log [H 3 0 + ] pH = - log (7.35 X 10 -13 ) pH = 12.13
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• Sample Problem C, pg. 545 Calculating [H 3 O + ] and [OH - ] from pH What are the concentrations of the hydronium and hydroxide ions in a sample of rain water that has a pH of 5.05? [H 3 O + ] = 10 pH [H 3 O + ] = 10 -5.05 [H 3 O + ] = 8.91 X 10 -6 M K w = [H 3 O + ] [OH - ] = 1.00 X 10 -14 8.91 X 10 -6 M [OH-] = 1.00 X 10 -14 [OH - ] = 1.12 X 10 -9 M
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• Additional Practice If the pH of a solution is 8.1, what is the [H 3 O + ] ? What is the [OH - ] in the solution? [H 3 O + ] = 10 pH [H 3 O + ] = 10 -8.1 [H 3 O + ] = 7.9 X 10 -9 M K w = [H 3 O + ] [OH - ] = 1.00 X 10 -14 7.9 X 10 -9 M [OH - ] = 1.00 X 10 -14 [OH - ] = 1.3 X 10 -6 M
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• Indicators Certain dyes, known as indicators, turn different colors in solutions of different pH. An indicator is a compound that can reversibly change color depending on the pH of the solution or other chemical change.
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• pH Meters A pH meter is an electronic instrument equipped with a probe that can be dipped into a solution. The probe has two electrodes, one of which is sensitive to the hydronium ion. An electrical voltage develops between the two electrodes, and the circuitry measures this voltage. The instrument converts the measurement into a pH reading, which is displayed on the meters screen.
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• Indicators vs. pH Meters Indicators Quick and convenient Do not give very precise results pH Meters Very precise More complicated and expensive
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• Homework Section 15.2 review, pg. 547: #1-3, 5, 7-12 Concept Review: Acidity, Basicity, and pH Be ready for a quiz over this section next time

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